1. Field of the Invention
The present invention relates to an optical pickup that projects a laser beam to an optical disc for reproducing or recording data, in particular an optical pickup including lenses having different numerical apertures for recording or reproducing a CD medium, a DVD medium and a BD medium.
2. Description of Related Art
An optical disc apparatus performs recording or reproducing of data by projecting a laser beam from an optical pickup to an optical disc that is a recording medium. If the optical disc apparatus can reproduce a CD (Compact Disc) medium and a DVD (Digital Versatile Disc) medium, the optical pickup is equipped with a laser light source that emits an infrared laser for a CD (approximately 780 nm) and a red laser for a DVD (approximately 650 nm). In addition, the optical pickup is equipped with an objective lens that condenses the laser beam to be a laser spot projected on a recording surface of the optical disc. A diameter of the laser spot is proportional to a wavelength of the laser beam and is inversely proportional to a numerical aperture (NA) of the objective lens. Since a CD medium and a DVD medium have different track widths, they need different objective lenses. However, it is possible that a single objective lens supports both CD media and DVD media by changing a shape of the objective lens.
Furthermore, these days, a BD (Blu-Ray Disc) having more storage capacity than the DVD has become available. Since a BD medium has higher recording density than a DVD medium, a track width of a BD medium is much smaller than a track width of a DVD medium. Therefore, a laser spot for a BD has to be smaller than that for a DVD. Therefore, a laser light source that emits blue laser having a shorter wavelength (approximately 405 nm) than the infrared laser or the red laser is used for recording and reproducing data on a BD medium.
In addition, the optical pickup described above detects reflection light from the optical disc, so that a position of the laser spot on the optical disc can be detected or data can be read. Therefore, it is necessary to project the laser beam to the optical disc so that an angle between the optical axis of the laser beam and the recording surface of the optical disc (tilt angle) becomes normally a constant angle (a right angle in many cases).
For this reason, there is proposed a structure in which an intermediate member is disposed between the objective lens and a lens holder to which the objective lens is attached in order to adjust an angle of the objective lens. The lens holder retains the intermediate member, and the contacting surface of the lens holder has substantially a spherical shape. In addition, a surface of the lens holder that contacts with the intermediate member has a conical shape or a stepped shape. An angle of the objective lens can be adjusted by rotating the intermediate member (see JP-A-H5-101429 and JP-A-H7-220286).
In addition, there is proposed a structure in which an objective lens has a spherical salient portion, a lens holder has a recess of a spherical shape that receives the spherical salient portion, the salient portion of the objective lens is arranged to engage with the recess of the lens holder, and the objective lens is moved to slide for adjusting the optical axis (see JP-A-7-220286, JP-A-2001-4891 and the like).
On the other hand, the objective lens for a CD and a DVD cannot converge the blue laser sufficiently because its NA is small, so it cannot record or reproduce a BD medium correctly. Further, if the objective lens for a BD is used, it cannot make the infrared laser or the red laser converge appropriately. Therefore, there is proposed a structure in which two objective lenses having different numerical apertures, i.e., the objective lens for a CD and a DVD and the objective lens for a BD are disposed on the lens holder that holds the objective lenses.
In this way, using the optical pickup equipped with two objective lenses having different numerical apertures, appropriate laser spots can be converged on the recording surfaces of the optical discs having different track widths so that recording and reproducing can be performed accurately (see JP-A-2006-19001 and the like).
Since the objective lens for recording and reproducing a BD medium has a short focal length so that its working distance is shorter than that is the objective lens for recording and reproducing a CD medium and a DVD medium, the objective lens for a BD medium is disposed at a position closer to the optical disc than the objective lens for a CD and a DVD in the objective lens holder to which the both objective lenses are attached.
However, in the conventional optical pickup having two objective lenses as described above, the working distance of the objective lens for a BD is short. For this reason, there may be a case where the objective lens for a BD and/or the objective lens holder contact with the optical disc medium (a CD medium, a DVD medium or a BD medium) when objective lens holder moves in the focusing direction for pulling in focusing or the like. If the objective lens and/or the objective lens holder contact with the optical disc medium, the optical pickup or the optical disc medium may be damaged and cause a malfunction. In addition, if there is a mismatching state of the tilt angle between the two objective lenses, coma aberration occurs so that performance of the optical pickup is lowered.
The coma aberration can be suppressed by adopting a mechanism for adjusting a tilt angle of the objective lens that is used in the optical pickup disclosed in JP-A-H5-101429, JP-A-H7-220286 or JP-A-2001-4891. However, when the tilt angle of the objective lens is adjusted, there will be a part in which a distance between the objective lens and/or the objective lens holder and the optical disc medium becomes shorter than the working distance compared with the case where the objective lens is attached normally to the objective lens holder. Then, the objective lens and/or the objective lens holder may contact with the optical disc medium, which may cause a malfunction or damage to the optical pickup and/or the optical disc medium.